Modern microwave radar and communication systems rely heavily on the performance characteristics of antennas. Continuing advancements in antenna design have resulted in improvements in these systems and have challenged instrumentation manufacturers to provide instruments that can accurately measure the performance characteristics of new antenna designs, such as multibeam antennas, electronically steered phased arrays, dual-polarized and shaped-beam reflectors, and low sidelobe and broadband omnidirectional antennas.
Antenna testing is a multidemensional measurement problem. The received signal from an antenna under test ("AUT") is a phasor quantity. It is a function of the antenna's frequency, beam position for steered-beam arrays, polarization, spatial position (such as azimuth and elevation angles), and other parameters. The received signal's amplitude and phase components are principal parameters of operation which are recorded as a function of position and other pertinent test parameters such as frequency and polarization.
It is believed that improvements in antenna range productivity may best be achieved by enhancing receiver performance characteristics. The major receiver characteristics affecting antenna measurements include measurement speed, frequency agility (the ability to change frequencies rapidly); dynamic range, interface compatibilities with computer-controlled systems, the number of measurement channels, and remote control capability.
The time required to take an individual measurement is governed primarily by a measurement receiver's acquisition speed. Measurement speed is the the rate at which a receiver can acquire, process, and output formatted data. The apparent speed of many conventional receivers is usually fast enough that the measurement processing time is transparent to an operator. However, actual receiver speed becomes apparent when repetitive tasks are performed, such as in multifrequency or multiported measurements wherein the frequency can be changed more rapidly than measurements can be processed and provided as outputs.
Fast, coherent, wideband measurements require that a receiver remain phase-locked to the transmit source at all times. A receiver's frequency agility characterizes its ability to maintain phase-lock when encountering rapid frequency transitions.
Receivers must also provide sufficient dynamic range for the required measurement accuracy. Current ultra-low sidelobe antennas have sidelobes in the -60 dB range, which may require a dynamic range of 80 dB or more for acceptable accuracy. When measurement speed is an overriding factor, the receiver's dynamic range may be compromised in exchange for speed. Also, receivers require interfacing to other measurement subsystems to produce outputs such as antenna radiation patterns.
Particular problems have been encountered in the design of measurement receivers which provide an acceptable response time, frequency agility, and dynamic range. For example, it is known that the signal level can suddently change in a measurement environment, as when the transmit antenna's polarization shifts from horizontal to vertical polarization. The sudden signal level change can result in overranging of the circuitry and inaccurate measurements unless the dynamic range and response time are sufficient to handle the change and can change quickly enough to preserve the measurement opportunity.
One known method for adapting to signal level changes may be referred to as the "dual-cycle" autoranging approach. In this method, the signal level from the previous cycle is used to set the gain level for a current cycle. This approach has the inherent disadvantage that there is risk of loss of data if the signal level changes abruptly between measurement cycles.
To avoid the latency resultant in dual-cycle autoranging, another known autoranging approach sets the gain stage for each measurement cycle based upon the incoming signal level. This method, which may be termed the "long-cycle" approach, involves lengthening the measurement cycle to allow for settling of the signals through the circuitry. Thus, this method has the disadvantage that the measurement cycle is necessarily lengthened, which affects overall measurement speed and bandwidth.
Calibration of measurement receivers is also of a critical importance in antenna testing. Calibration of a receiver entails providing a reference signal (which may originate in a local oscillator unit associated with a test range), typically at full scale and at a known phase, and measuring the effect which the various circuits in the receiver may have on the reference signal. Corrections for detected errors are then provided. During calibration, the reference signal is typically imposed upon both the signal channel and the reference channel of the receiver, both of which must be independently calibrated. In addition to measuring gain effects, the phase of the signals in both channels must be measured and corrected, if necessary, so as to cause measurement values during operation to be expressed relative to a calibrated signal. Often, measurement values are expressed in complex signal form, with an inphase component (I) and a quadrature component (Q), which are 90.degree. out of phase with respect to each other.
In particular, three error types typically occur in phase/amplitude measurement receivers utilizing complex signal detection circuits: (1) offset errors, that is, fixed values which must be added to the inphase or quadrature outputs of the receiver channel, (2) orthogonality errors, that is, deviation of the inphase and quadrature reference signals from being exactly 90.degree. out of phase with respect to each other, or deviation of the phase shifts in the inphase and quadrature sides of the demodulator from each other, and (3) gain errors, that is, a discrepancy in the gain of the inphase signal path as compared to the quadrature signal path.
In one known prior art approach to the calibration problem for a complex signal measurement circuit, U.S. Pat. No. 3,950,750 to Churchill et al., a calibration reference signal is generated with a slight frequency offset, causing its phase to continuously vary during the calibration process. The phasor output of the complex signal circuit is sampled at appropriate intervals, and a fast Fourier transform (FFT) is performed on the resulting data. The result of the FFT is used to generate correction coefficients for application to signals passing through the complex signal circuit. However, this approach relies upon precise timing of the samples with respect to the phase of the frequency offset reference signal, plus the added computational complexity attendant to the FFT algorithm.
Another similar prior art approach eliminates the timing constraints by using switchable phase shifting elements within a calibration reference signal generator. However, this technique relies heavily upon the precision and constancy of the phase shifting hardware. Controllable phase shifts of the precision required for accurate antenna measurement applications are difficult and expensive to achieve.
Accordingly, there is still need for further improvements in radio frequency receivers used in radar, antenna, components, and materials testing, and communications applications, namely, in methods for calibration, complex signal generation, autoranging, and the like, at lower cost but with improved performance.